Method and device for the laser-based machining of sheet-like substrates

ABSTRACT

A method for the laser-based machining of a sheet-like substrate, in order to separate the substrate into multiple portions, in which the laser beam of a laser for machining the substrate is directed onto the latter, is characterized in that, with an optical arrangement positioned in the path of rays of the laser, an extended laser beam focal line, seen along the direction of the beam, is formed on the beam output side of the optical arrangement from the laser beam directed onto the latter, the substrate being positioned in relation to the laser beam focal line such that an induced absorption is produced in the material of the substrate in the interior of the substrate along an extended portion, seen in the direction of the beam, of the laser beam focal line, such that a material modification takes place in the material of the substrate along this extended portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/752,489 filed on Jan. 15, 2013,and claims priority under 35 U.S.C. §119 or 365 to European ApplicationNo. EP 13 151 296, filed Jan. 15, 2013, the content of which is reliedupon and incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates generally to a method for the laser-basedmachining of preferably sheet-like substrates and to a correspondingdevice and to the use of methods and devices for separating sheet-likesubstrates, such as for example semiconductor wafers, glass elements, .. . (in particular of brittle materials) into multiple parts(individually separating the wafers or glass elements). As furtherdescribed in detail below, work is in this case performed using lasers,generally pulsed lasers, with a wavelength to which the materials aresubstantially transparent.

Devices and methods for severing such materials by means of a laser arealready known from the prior art. On the one hand (for example DE 102011 000 768 A1), it is possible to use lasers which, by virtue of theirwavelength or their power, are strongly absorbed by the material, orafter the first interaction make the material strongly absorbent(heating with for example the generation of charge carriers; inducedabsorption), and can then ablate the material. This method hasdisadvantages in the case of many materials: for example impurities dueto particle formation in the ablation; cut edges may have microcracks onaccount of the heat input; cut edges may have melt edges; the cuttinggap is not uniform over the thickness of the material (has a differentwidth at different depths; for example a wedge-shaped cutting notch).Since material has to be vaporized or liquefied, a high average laserpower has to be provided.

On the other hand, there are known laser methods for severing brittlematerials that function on the basis of a specifically directed,laser-induced crack formation. For example, there is a method fromJenoptik in which a trace on the surface is first strongly heated by thelaser, and immediately thereafter this trace is cooled so quickly (forexample by means of a water jet) that the thermal stresses therebyachieved lead to crack formation, which may be propagated through thethickness of the material (mechanical stress) in order to sever thematerial.

There are also methods in which a laser at the wavelength of which thematerial is largely transparent is used, so that a focal point can beproduced in the interior of the material. The intensity of the lasermust be so high that internal damage takes place at this internal focalpoint in the material of the irradiated substrate.

The last-mentioned methods have the disadvantage that the induced crackformation takes place in the form of a point at a specific depth, or onthe surface, and so the full thickness of the material is only severedby way of an additional, mechanically and/or thermally induced crackpropagation. Since cracks tend to spread unevenly, the separatingsurface is usually very rough and must often be re-worked. Moreover, thesame process has to be applied a number of times at different depths.This in turn slows down the speed of the process by the correspondingmultiple.

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

An object of the present invention is therefore to provide a method (anda corresponding device) with which sheet-like substrates, in particularof brittle materials, can be machined, in particular completely severed,without significant particle formation, without significant melt edges,with minimal crack formation at the edge, without significant cuttinggaps (that is to say material losses), with straightest-possible cutedges and with a high speed of the process.

One embodiment of the disclosure relates to a method that includesfocusing a pulsed laser beam into a laser beam focal line, viewed alongthe beam propagation direction, the laser beam focal line having alength in a range of between 0.1 mm and 100 mm, and directing the laserbeam focal line into a material at an angle of incidence to a surface ofthe material, the laser beam focal line generating an induced absorptionwithin the material, the induced absorption producing a materialmodification along the laser beam focal line within the material.

An additional embodiment of the disclosure relates to a system thatincludes a pulsed laser and an optical assembly positioned in the beampath of the laser, configured to transform the laser beam into an laserbeam focal line, viewed along the beam propagation direction, on thebeam emergence side of the optical assembly, the laser beam focal linehaving a length in a range of between 0.1 mm and 100 mm, the opticalassembly including a focusing optical element with spherical aberrationconfigured to generate the laser beam focal line, said laser beam focalline adapted to generate an induced absorption within a material, theinduced absorption producing a material modification along the laserbeam focal line within the material.

Another embodiment of the disclosure relates to a glass article thatincludes at least one surface having a plurality of materialmodifications along the surface, each material modification having alength in a range of between 0.1 mm and 100 mm, and an average diameterin a range of between 0.5 μm and 5 μm. Yet another embodiment of thedisclosure relates to a glass article comprising at least one surfacehaving a plurality of material modifications along the surface, eachmaterial modification having a ratio V3=a/δ of the average distance a ofthe directly neighboring material modifications and the average diameterδ of a laser beam focal line that created the material modificationsequal to approximately 2.0.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between the heat diffusion constant α, thelinear extent in the material (scale length, denoted here by d) and atime duration τ, such as for example the laser pulse duration forvarious materials.

FIG. 2 shows the principle of the positioning of a focal line, that isto say the machining of a material that is transparent to the laserwavelength, on the basis of the induced absorption along the focal line.

FIG. 3 a shows a first optical arrangement that can be used inembodiments described herein.

FIG. 3 b shows various possible ways of machining the substrate bydifferent positioning of the laser beam focal line in relation to thesubstrate.

FIG. 4 shows a second optical arrangement that can be used inembodiments described herein.

FIGS. 5 a and 5 b show a third optical arrangement that can be used inembodiments described herein.

FIG. 6 shows a fourth optical arrangement that can be used inembodiments described herein.

FIG. 7 shows a setup for carrying out the method in the example of thefirst usable optical arrangement from FIG. 3 a (instead of this opticalarrangement, the further optical arrangements shown in FIGS. 4, 5 and 6may also be used within the scope of the arrangement shown, in that theoptical arrangement 6 shown in FIG. 7 is replaced by one of thesearrangements).

FIG. 8 shows the production of a focal line in detail.

FIG. 9 shows a micrograph of the surface (plan view of the plane of thesubstrate) of a glass sheet machined as described herein.

DETAILED DESCRIPTION

Various embodiments will be further clarified by the following examples.

One embodiment of the disclosure relates to a method that includesfocusing a pulsed laser beam into a laser beam focal line, viewed alongthe beam propagation direction, the laser beam focal line having alength in a range of between 0.1 mm and 100 mm, and directing the laserbeam focal line into a material at an angle of incidence to a surface ofthe material, the laser beam focal line generating an induced absorptionwithin the material, the induced absorption producing a materialmodification along the laser beam focal line within the material. Themethod can further include translating the material and the laser beamrelative to each other, thereby producing a plurality of materialmodifications within the material, the material modifications spacedapart so as to separate the material into at least two pieces. The laserbeam can have an average laser energy measured at the material less thanabout 400 μJ, such as less than about 250 μJ. The pulse duration can bein a range of between greater than about 10 picoseconds and less thanabout 100 picoseconds, or less than 10 picoseconds. The pulse repetitionfrequency can be in a range of between 10 kHz and 1000 kHz, such as in arange of between 10 kHz and 100 kHz, or less than 10 kHz. The materialcan be glass, sapphire, a semiconductor wafer, or the like. Materialmodification can be crack formation. The angle of incidence of the laserbeam focal line can be less than or equal to about 45 degrees to thesurface of the material, such as perpendicular to the surface of thematerial. The laser beam focal line can be contained entirely within thematerial, with the laser beam focal line not extending to either surfaceof the material. The material modification can extend within thematerial to at least one of two opposing surfaces of the material, suchas extending within the material from one of two opposing surfaces ofthe material to the other one of the two opposing surfaces, over theentire thickness of the material. In particular, for each laser pulse,the material modification can extend within the material from one of twoopposing surfaces of the material to the other one of the two opposingsurfaces, over the entire thickness of the material. The pulsed laserbeam can have a wavelength selected such that the material issubstantially transparent at this wavelength. The wavelength can be lessthan about 1.8 μm. The laser beam focal line can have an average spotdiameter in a range of between 0.5 μm and 5 μm.

An additional embodiment of the disclosure relates to a system thatincludes a pulsed laser and an optical assembly positioned in the beampath of the laser, configured to transform the laser beam into an laserbeam focal line, viewed along the beam propagation direction, on thebeam emergence side of the optical assembly, the laser beam focal linehaving a length in a range of between 0.1 mm and 100 mm, the opticalassembly including a focusing optical element with spherical aberrationconfigured to generate the laser beam focal line, said laser beam focalline adapted to generate an induced absorption within a material, theinduced absorption producing a material modification along the laserbeam focal line within the material. The laser energy, pulse duration,pulse repetition frequency, wavelength, focal line diameter, material,and material modification for the system can be as described above forthe method. The optical assembly can include an annular aperturepositioned in the beam path of the laser before the focusing opticalelement, the annular aperture configured to block out one or more raysin the center of the laser beam so that only marginal rays outside thecenter incide on the focusing optical element, and thereby only a singlelaser beam focal line, viewed along the beam direction, is produced foreach pulse of the pulsed laser beam. The focusing optical element can bea spherically cut convex lens, such as a conical prism having anon-spherical free surface, such as an axicon. The optical assembly canfurther include a second optical element, the two optical elementspositioned and aligned such that the laser beam focal line is generatedon the beam emergence side of the second optical element at a distancefrom the second optical element.

Another embodiment of the disclosure relates to a glass article thatincludes at least one surface having a plurality of materialmodifications along the surface, each material modification having alength in a range of between 0.1 mm and 100 mm, and an average diameterin a range of between 0.5 μm and 5 μm. Yet another embodiment of thedisclosure relates to a glass article comprising at least one surfacehaving a plurality of material modifications along the surface, eachmaterial modification having a ratio V3=a/δ of the average distance a ofthe directly neighboring material modifications and the average diameterδ of a laser beam focal line that created the material modificationsequal to approximately 2.0.

The present disclosure is described below, at first generally, then indetail on the basis of several exemplary embodiments. The features shownin combination with one another in the individual exemplary embodimentsdo not all have to be realized. In particular, individual features mayalso be omitted or combined in some other way with other features shownof the same exemplary embodiment or else of other exemplary embodiments.It is also possible that individual features of one exemplary embodimentalready in themselves display advantageous developments of the priorart.

The mechanism of separating the substrate into individual parts is firstdescribed below.

The separating method produces for each laser pulse a laser focal line(as distinct from a focal point) by means of laser optics suitabletherefor (hereinafter also referred to as an optical arrangement). Thefocal line determines the zone of the interaction between the laser andthe material of the substrate. If the focal line falls in the materialto be separated, the laser parameters can be chosen such that aninteraction with the material which produces a crack zone along thefocal line takes place. Important laser parameters here are thewavelength of the laser, the pulse duration of the laser, the pulseenergy of the laser and possibly also the polarization of the laser.

The following should preferably be provided for the interaction of thelaser light with the material:

1) The wavelength 1 of the laser is preferably chosen such that thematerial is substantially transparent at this wavelength (specificallyfor example: absorption <<10% per mm of material depth=>γ<<1/cm; γ:Lambert-Beer absorption coefficient).

2) The pulse duration of the laser is preferably chosen such that nosignificant heat transport (heat diffusion) out of the zone ofinteraction can take place within the time of interaction (specificallyfor example: τ<<d²/α, d: focus diameter, τ: laser pulse duration, α:heat diffusion constant of the material).

3) The pulse energy of the laser is preferably chosen such that theintensity in the zone of interaction, that is to say in the focal line,produces an induced absorption, which leads to local heating of thematerial along the focal line, which in turn leads to crack formationalong the focal line as a result of the thermal stress introduced intothe material.

4) The polarization of the laser influences both the interaction at thesurface (reflectivity) and the type of interaction within the materialin the induced absorption. The induced absorption may take place by wayof induced, free charge carriers (typically electrons), either afterthermal excitation, or by way of multiphoton absorption and internalphotoionization, or by way of direct field ionization (field strength ofthe light breaks electron bonding directly). The type of generation ofthe charge carriers can be assessed for example by way of the so-calledKeldysh parameter, which however does not play any role for theapplication of the method. In the case of certain materials (for examplebirefringent materials) it may just be important that the furtherabsorption/transmission of the laser light depends on the polarization,and consequently the polarization by way of suitable optics (phaseplates) should be chosen by the user to be conducive for separating therespective material, for example simply in a heuristic way. Therefore,if the material is not optically isotropic, but for examplebirefringent, the propagation of the laser light in the material is alsoinfluenced by the polarization. Thus, the polarization and theorientation of the polarization vector may be chosen such that, asdesired, there only forms one focal line and not two (ordinary andextraordinary rays). In the case of optically isotropic materials, thisdoes not play any role.

5) Furthermore, the intensity should be chosen on the basis of the pulseduration, the pulse energy and the focal line diameter such that thereis preferably no significant ablation or significant melting, butpreferably only crack formation in the microstructure of the solid body.For typical materials such as glass or transparent crystals, thisrequirement can be satisfied most easily with pulsed lasers in thesub-nanosecond range, that is to say in particular with pulse durationsof for example between 10 and 100 ps. In this respect, also see FIG. 1:over scale lengths of approximately one micrometer (0.5 to 5.0micrometers, cf. center of image), for poor heat conductors, such asglasses for example, the heat conduction acts into the sub-microsecondrange (see the range between the two lines), while for good heatconductors, such as crystals and semiconductors, the heat conduction isalready effective within nanoseconds.

The essential process for the crack formation in the material occurring,and made to extend vertically to the plane of the substrate, ismechanical stress that exceeds the structural strength of the material(compressive strength in MPa). The mechanical stress is achieved here byway of rapid, inhomogeneous heating (thermally induced stress) by thelaser energy. Presupposing appropriate positioning of the substrate inrelation to the focal line (see below), the crack formation starts ofcourse at the surface of the substrate, since that is where thedeformation is greatest. The reason for this is that in the half-spaceabove the surface there is no material that can absorb forces. Thisargument also applies to materials with hardened or toughened surfaces,as long as the thickness of the hardened or toughened layer is great incomparison with the diameter of the abruptly heated material along thefocal line. In this respect also see FIG. 2, further described below.

The type of interaction can be set by way of the fluence (energy densityin Joules per cm²) and the laser pulse duration with a selected focalline diameter such that preferably 1.) no significant melting takesplace at the surface or in the volume and 2.) no significant ablationwith particle formation takes place at the surface. In the substantiallytransparent materials, several types of induced absorption are known:

a) In semiconductors and isolators with a low band gap, on the basis forexample of a low residual absorption (due to traces of impurities in thematerial or due to charge carriers already thermally excited at thetemperature before the laser machining), rapid heating up within a firstfraction of the laser pulse duration will lead to thermal excitation offurther charge carriers, which in turn leads to increased absorption andconsequently to a cumulative increase in the laser absorption in thefocal line.

b) In isolators, if there is sufficiently high light intensity, a photoabsorption leads to an ionization on the basis of a nonlinear-opticalinteraction with the atoms of the material, and consequently in turn tothe generation of free charge carriers, and consequently to increasedlinear absorption of the laser light.

The production of the geometry of a desired separating surface (relativemovement between the laser beam and the substrate along a line on thesubstrate surface) is described below.

The interaction with the material produces for each laser pulse anindividual, continuous (seen in the direction perpendicular to thesubstrate surface) crack zone in the material along a focal line. Forthe complete severing of the material, a series of these crack zones foreach laser pulse is set so close together along the desired separatingline that a lateral connection of the cracks produces a desired cracksurface/contour in the material. For this, the laser is pulsed at aspecific repetition rate. The spot size and spacing are chosen such thata desired, directed crack formation occurs at the surface, along theline of the laser spots. The spacing of the individual crack zones alongthe desired separating surface is obtained from the movement of thefocal line in relation to the material within the time period from laserpulse to laser pulse. In this respect, also see FIG. 9, furtherdescribed below.

To produce the desired separating surface in the material, either thepulsed laser light may be moved over the stationary material by anoptical arrangement that is movable parallel to the plane of thesubstrate (and possibly also perpendicularly thereto), or the materialitself is moved with a movable holder past the stationary opticalarrangement such that the desired separating line is formed. Theorientation of the focal line in relation to the surface of thematerial, whether perpendicular or at an angle of 90°-β to the surface,may either be chosen as a fixed value or be changed by way of apivotable optical arrangement (hereinafter also referred to forsimplicity as optics) and/or a pivotable beam path of the laser alongthe desired separating line.

Altogether, for forming the desired separating line, the focal line maybe passed through the material in up to five separately movable axes:two spatial axes (x, y), which fix the point of penetration of the focalline into the material, two angular axes (theta, phi), which fix theorientation of the focal line from the point of penetration into thematerial, and a further spatial axis (z′, not necessarily orthogonal tox, y), which fixes how deep the focal line reaches into the materialfrom the point of penetration at the surface. For the geometry in theCartesian system of coordinates (x, y, z), also see for example FIGS. 5a and 6, described below. In the case of perpendicular incidence of thelaser beam on the substrate surface (β=0°, then z=z′.

There are generally restrictions here, dictated by the optics and thelaser parameters: the orientation of the angles in theta and phi canonly take place to the extent that the refraction of the laser light inthe material allows (less than the angle of total reflection in thematerial), and the depth of penetration of the laser focal line isrestricted by the available laser pulse energy and the accordinglychosen laser optics, which only forms a length of the focal line thatcan produce the crack zone with the laser pulse energy available.

One possible configuration for moving the focal lines in all five axesmay for example comprise moving the material on a driven axial table inthe coordinates x, y, while by way of a galvoscanner and anon-telecentric F-theta lens the focal line is moved in the field of thelens in relation to the center of the lens in the coordinates x′, y′ andis tilted by the angles theta, phi. The coordinates x and x′ and y andy′ may be calculated such that the focal line is directed at the desiredpoint of impingement of the surface of the material. The galvoscannerand F-theta lens are also fastened on a z axis, which is orthogonal tothe x,y plane of the axial table and determines the position of thefocal line perpendicularly to the material (depth of the focal line inthe material).

The last step of separating the substrate into the multiple parts isdescribed below (separation or individual separation).

The separation of the material along the crack surface/contour producedtakes place either by internal stress of the material or by forcesintroduced, for example mechanically (tension) or thermally (unevenheating/cooling). Since, preferably, no significant amount of materialis ablated, there is generally initially no continuous gap in thematerial, but only a highly disturbed fracture surface area(microcracks), which is meshed within itself and under somecircumstances still connected by bridges. The forces subsequentlyintroduced have the effect of separating the remaining bridges andovercoming the meshing by way of lateral crack growth (taking placeparallel to the plane of the substrate), so that the material can beseparated along the separating surface.

Additional embodiments of a method and of a device are described below.

In one embodiment, a method for the laser-based machining of apreferably sheet-like substrate (1), in particular a wafer or glasselement, in order to separate the substrate into multiple parts, inwhich the laser beam (2 a, 2 b) of a laser (3) for machining thesubstrate (1) is directed onto the latter, is characterized in that withan optical arrangement (6) positioned in the path of rays of the laser(3), an extended laser beam focal line (2 b), seen along the directionof the beam, is formed on the beam output side of the opticalarrangement (6) from the laser beam (2 a) directed onto the latter, thesubstrate (1) being positioned in relation to the laser beam focal line(2 b) such that an induced absorption is produced in the material of thesubstrate (1) along an extended portion (2 c), seen in the direction ofthe beam, of the laser beam focal line (2 b), with the effect that aninduced crack formation takes place in the material of the substratealong this extended portion (2 c).

In some embodiments, the substrate (1) is positioned in relation to thelaser beam focal line (2 b) such that the extended portion (2 c) of theinduced absorption in the material, that is to say in the interior ofthe substrate (1), extends up to at least one of the two oppositesubstrate surfaces (1 a, 1 b).

In certain embodiments, the substrate (1) is positioned in relation tothe laser beam focal line (2 b) such that the extended portion (2 c) ofthe induced absorption in the material, that is to say in the interiorof the substrate (1), extends from one (1 a) of the two oppositesubstrate surfaces up to the other (1 b) of the two opposite substratesurfaces, that is to say over the entire layer thickness d of thesubstrate (1) or in that the substrate (1) is positioned in relation tothe laser beam focal line (2 b) such that the extended portion (2 c) ofthe induced absorption in the material, that is to say in the interiorof the substrate (1), extends from one (1 a) of the two oppositesubstrate surfaces into the substrate (1), but not up to the other (1 b)of the two opposite substrate surfaces, that is to say not over theentire layer thickness d of the substrate (1), preferably extends over80% to 98%, preferably over 85 to 95%, particularly preferably over 90%,of this layer thickness.

In some embodiments, the induced absorption is produced such that thecrack formation takes place in the microstructure of the substrate (1)without ablation and without melting of material of the substrate (1).

In certain embodiments, the extent 1 of the laser beam focal line (2 b)and/or the extent of the portion (2 c) of the induced absorption in thematerial, that is to say in the interior of the substrate (1), seen ineach case in the longitudinal direction of the beam, is between 0.1 mmand 100 mm, preferably between 0.3 mm and 10 mm, and/or in that thelayer thickness d of the substrate (1), measured perpendicularly to thetwo opposite substrate surfaces (1 a, 1 b), is between 30 μm and 3000μm, preferably between 100 μm and 1000 μm, and/or in that the ratioV1=l/d of this extent 1 of the laser beam focal line (2 b) and thislayer thickness d of the substrate (1) is between 10 and 0.5, preferablybetween 5 and 2, and/or in that the ratio V2=L/D of the extent L of theportion (2 c) of the induced absorption in the material, that is to sayin the interior of the substrate (1), seen in the longitudinal directionof the beam, and the average extent D of the portion (2 c) of theinduced absorption in the material, that is to say in the interior ofthe substrate (1), seen transversely to the longitudinal direction ofthe beam, is between 5 and 5000, preferably between 50 and 500.

In some embodiments, the average diameter δ of the laser beam focal line(2 b), that is to say the spot diameter, is between 0.5 μm and 5 μm,preferably between 1 μm and 3 μm, preferably is 2 μm, and/or in that thepulse duration τ of the laser (3) is chosen such that, within the timeof interaction with the material of the substrate (1), the heatdiffusion in this material is negligible, preferably no heat diffusiontakes place, for which preferably τ, δ and the heat diffusion constant αof the material of the substrate (1) are set according to τ<<δ²/α and/orpreferably τ is chosen to be less than 10 ns, preferably less than 100ps, and/or in that the pulse repetition rate of the laser (3) is between10 kHz and 1000 kHz, preferably is 100 kHz, and/or in that the laser (3)is operated as a single-pulse laser or as a burst-pulse laser, and/or inthat the average laser power, measured directly on the output side ofthe beam of the laser (3), is between 10 watts and 100 watts, preferablybetween 30 watts and 50 watts.

In certain embodiments, the wavelength λ of the laser (3) is chosen suchthat the material of the substrate (1) is transparent to this wavelengthor is substantially transparent, the latter being understood as meaningthat the decrease in intensity of the laser beam taking place along thedirection of the beam in the material of the substrate (1) permillimeter of the depth of penetration is 10% or less, the laser being,in particular for glasses or crystals that are transparent in thevisible wavelength range as the substrate (1), preferably an Nd:YAGlaser with a wavelength λ of 1064 nm or a Y:YAG laser with a wavelengthλ of 1030 nm, or, in particular for semiconductor substrates (1) thatare transparent in the infrared wavelength range, preferably an Er:YAGlaser with a wavelength λ of between 1.5 μm and 1.8 μm.

In some embodiments, the laser beam (2 a, 2 b) is directedperpendicularly onto the substrate (1), in that therefore the substrate(1) is positioned in relation to the laser beam focal line (2 b) suchthat the induced absorption along the extended portion (2 c) of thelaser beam focal line (2 b) takes place perpendicularly to the plane ofthe substrate or in that the laser beam (2 a, 2 b) is directed onto thesubstrate (1) at an angle β of greater than 0° in relation to the normalto the plane of the substrate (1), in that therefore the substrate (1)is positioned in relation to the laser beam focal line (2 b) such thatthe induced absorption along the extended portion (2 c) of the laserbeam focal line (2 b) takes place at the angle 90°-β to the plane of thesubstrate, where preferably β≦45°, preferably β≦30°.

In certain embodiments, the laser beam (2 a, 2 b) is moved in relationto the surface (1 a, 4) of the substrate (1) along a line (5) alongwhich the substrate (1) is to be severed to obtain the multiple parts, amultiplicity (2 c-1, 2 c-2, . . . ) of extended portions (2 c) ofinduced absorption in the interior of the substrate (1) being producedalong this line (5), where preferably the ratio V3=a/δ of the averagespacing a of directly adjacent extended portions (2 c) of inducedabsorption, that is to say portions produced directly one after theother, and the average diameter 8 of the laser beam focal line (2 b),that is to say the spot diameter, is between 0.5 and 3.0, preferablybetween 1.0 and 2.0.

In some embodiments, during and/or after the production of themultiplicity (2 c-1, 2 c-2, . . . ) of extended portions (2 c) ofinduced absorption in the interior of the substrate (1), mechanicalforces are exerted on the substrate (1) and/or thermal stresses areintroduced into the substrate (1), in particular the substrate isunevenly heated and cooled again, in order to bring about crackformation for separating the substrate into the multiple partsrespectively between directly adjacent (2 c-1, 2 c-2) extended portions(2 c) of induced absorption, the thermal stresses preferably beingintroduced by irradiating the substrate (1) with a CO₂ laser along theline (5).

In some embodiments, a device for the laser-based machining of apreferably sheet-like substrate (1), in order to separate the substrateinto multiple parts, with which the laser beam (2 a, 2 b) of a laser (3)for machining the substrate (1) can be directed onto the latter, ischaracterized by an optical arrangement (6), which is positioned in thepath of rays of the laser (3) and with which an extended laser beamfocal line (2 b), seen along the direction of the beam, can be formed onthe beam output side of the optical arrangement (6) from the laser beam(2 a) directed onto the latter, the substrate (1) being able to bepositioned or being positioned in relation to the laser beam focal line(2 b) such that an induced absorption takes place in the material of thesubstrate (1) along an extended portion (2 c), seen in the direction ofthe beam, of the laser beam focal line (2 b), with the effect that aninduced crack formation is brought about in the material of thesubstrate along this extended portion (2 c).

In certain embodiments, the optical arrangement (6) comprises a focusingoptical element with spherical aberration, preferably a sphericallyground convex lens (7), a diaphragm (8) of the optical arrangement (6),preferably an annular diaphragm, preferably being positioned before thisfocusing optical element (7) in the path of rays of the laser (3), withthe effect that the bundle of rays (2 aZ) lying at the center of thelaser beam (2 a) impinging onto the diaphragm can be blocked out, sothat only the peripheral rays (2 aR) lying outside this center impingeonto this focusing optical element.

In some embodiments, the optical arrangement (6) comprises an opticalelement with a non-spherical free surface which is shaped for formingthe laser beam focal line (2 b) with a defined extent 1, that is to saya defined length, seen in the direction of the beam, the optical elementwith the non-spherical free surface preferably being a cone prism or anaxicon (9).

In certain embodiments, the optical arrangement (6) comprises in thepath of rays of the laser (3) firstly a first optical element with anon-spherical free surface, which is shaped for the forming of theextended laser beam focal line (2 b), preferably a cone prism or anaxicon (10), and, on the beam output side of this first optical elementand at the distance z1 from it, a second, focusing optical element, inparticular a convex lens (11), these two optical elements preferablybeing positioned and aligned such that the first optical elementprojects the laser radiation impinging on it annularly (SR) onto thesecond optical element, so that the extended laser beam focal line (2 b)is produced on the beam output side of the second optical element at thedistance z2 from it.

In some embodiments, a third, focusing optical element, which is inparticular a plano-convex collimation lens (12), is positioned betweenthe first and second optical elements in the path of rays of the laser(3), the third optical element preferably being positioned and alignedsuch that the laser radiation formed annularly (SR) by the first opticalelement falls onto the third optical element with a defined average ringdiameter dr and in that the third optical element projects the laserradiation annularly with this ring diameter dr and with a defined ringwidth br onto the second optical element.

The methods or devices described above can be used for separatingsubstrates of glass, in particular of quartz, borosilicate, sapphire orsoda-lime glass, sodium-containing glass, hardened glass or unhardenedglass, of crystalline Al₂O₃, of SiO₂.nH₂O (opal) or of a semiconductormaterial, in particular Si, GaAs, GaN, separating single- ormulti-layered substrates, in particular glass-glass composites,glass-film composites, glass-film-glass composites or glass-air-glasscomposites, separating coated substrates, in particular metal-coatedsapphire wafers, silicon wafers provided with metal or metal-oxidelayers or substrates coated with ITO or AlZnO, and/or completelysevering a single- or multi-layered substrate or severing one or more,but not all of the layers of a multi-layered substrate.

The laser beam focal line produced by means of the optical arrangementdescribed above is alternatively also referred to above and below forsimplicity as the focal line of the laser beam. The substrate isseparated or individually separated into the multiple parts, seen in theplane of the substrate, by the crack formation (induced absorption alongthe focal line made to extend perpendicularly to the plane of thesubstrate). The crack formation consequently takes place perpendicularlyto the plane of the substrate into the substrate or into the interior ofthe substrate (longitudinal crack formation). As already described,generally a multiplicity of individual laser beam focal lines have to beintroduced into the substrate along a line on the substrate surface, inorder that the individual parts of the substrate can be separated fromone another. For this purpose, either the substrate may be made to moveparallel to the plane of the substrate in relation to the laser beam orin relation to the optical arrangement or, conversely, the opticalarrangement may be moved parallel to the plane of the substrate inrelation to the fixedly arranged substrate.

The features of at least one of the dependent method or device claimsare advantageously additionally realized. In this respect, the featuresof a number of dependent claims may also be realized in any desiredcombination.

In one particular aspect, the extended portion of the induced absorptionin the interior of the substrate extends from a surface of the substrateto a defined depth of the substrate (or even beyond). The extendedportion of the induced absorption may in this case comprise the entiredepth of the substrate from one surface to the other. It is alsopossible to produce longitudinally extended portions of the inducedabsorption only in the interior of the substrate (without including thesurfaces of the substrate).

Further features that can be advantageously realized can be seen in FIG.3 b further described below. The extended portion of the inducedabsorption (that is to say for example crack length introducedperpendicularly to the plane of the substrate) can consequently extendboth from a point in the interior of the substrate along the extendedportion of the induced absorption to the rear surface of the substrateor else for example from the front surfaces of the substrate to a pointin the interior of the substrate. The layer thickness d is in this caserespectively measured perpendicularly to the two opposite substratesurfaces of the sheet-like substrate (even in the case where the laserradiation is directed obliquely at an angle β>0° to the normal to thesubstrate surface, that is to say in the case of oblique incidence).

As used herein, the range limits mentioned in each case include theupper and lower limit values indicated.

The induced absorption is advantageously produced by means of thesetting of the already described laser parameters, which are alsoexplained below within the scope of examples, the parameters of theoptical arrangement, and the geometrical parameters of the arrangementof the individual elements of the device. In principle, any desiredcombination of features of parameters is possible here. For instance,τ<<δ²/α means here that τ is less than 1%, preferably less than 1%, ofδ²/α. For example, the pulse duration τ may be 10 ps (or else belowthat), between 10 and 100 ps or else above 100 ps. For separating Sisubstrates, preferably an Er:YAG laser with a wavelength of between 1.5and 1.8 μm is used. For semiconductor substrates, generally a laser witha wavelength that is chosen such that the photon energy is less than theband gap of the semiconductor is preferably used.

Advantageous radiating directions for directing the laser beam onto thesubstrate (which then also define the orientation of the laser beamfocal line in relation to the plane of the substrate) include directingthe laser beam (2 a, 2 b) perpendicularly onto the substrate (1), inthat therefore the substrate (1) is positioned in relation to the laserbeam focal line (2 b) such that the induced absorption along theextended portion (2 c) of the laser beam focal line (2 b) takes placeperpendicularly to the plane of the substrate, or directing the laserbeam (2 a, 2 b) onto the substrate (1) at an angle β of greater than 0°in relation to the normal to the plane of the substrate (1), in thattherefore the substrate (1) is positioned in relation to the laser beamfocal line (2 b) such that the induced absorption along the extendedportion (2 c) of the laser beam focal line (2 b) takes place at theangle 90°-β to the plane of the substrate, where preferably β≦45°,preferably β≦30°.

The additional method steps that are possibly also necessary for thefinal separation or individual separation of the substrate into itsmultiple parts are described below. As already mentioned, either thesubstrate is moved in relation to the optical arrangement (together withthe laser) or the optical arrangement (together with the laser) is movedin relation to the substrate. The crack formation should in this case(by contrast with the induced crack formation described above) beunderstood as meaning a transverse crack, that is to say a lateral crackformation in a direction lying in the plane of the substrate(corresponding to the path of the line along which the substrate is tobe separated).

Further developments of a device, which describe in particular variouspossible configurational forms of the optical arrangement for producingand positioning the laser beam focal line, are described below. In thisrespect, also see the following exemplary embodiments and FIGS. 3 a, 4,5 a, 5 b, 6, 7 and 8. The convex lens may in particular be aplano-convex lens.

Main uses according to the invention (others are also described below)are described above.

A series of significant advantages in comparison with the methods anddevices that are known from the prior art are described below.

Firstly, according to the invention, the formation of the cut takesplace without significant particle formation, without significant meltedges, with minimal crack formation at the edge, without any significantcutting gap (consequently without loss of material of the substrate) andwith straight cut edges. The formation of the cut may in this case beset either perpendicularly (seen in relation to the plane of thesubstrate) or at an angle β desired by the user in relation to thenormal to the substrate.

In particular, not a very high average laser power is necessary, butnevertheless comparatively high separating speeds can be achieved. It isessential in this respect that for each laser pulse (or each burstpulse) a laser beam focal line is produced (and not just a focal pointof no extent, or only very local extent). The laser optics presented infurther detail below are used for this purpose. The focal line thusdetermines the zone of interaction between the laser and the substrate.If the focal line falls at least as a portion thereof (seen in the depthdirection) into the substrate material to be separated, the laserparameters can be chosen such that an interaction with the materialwhich produces a crack zone along the entire focal line (or along theentire extended portion of the laser beam focal line that falls into thesubstrate) takes place. Selectable laser parameters are, for example,the wavelength of the laser, the pulse duration of the laser, the pulseenergy of the laser and also possibly the polarization of the laser.

Further advantages that the method has in comparison for example withmechanical scoring and breaking are not only the absent (or at leastminimal) particle formation but also, by contrast with a mechanicalscoring line, that a high aspect ratio (width to depth) can be achieved.While in the case of mechanical scoring and breaking the rupture lineinto the material is produced by way of largely uncontrollable crackgrowth, according to the invention separation at a very preciselysettable angle β to the normal to the substrate takes place.Consequently, according to the invention, there is no directionaldependence of the cutting direction, and oblique cuts are readilypossible.

Also in comparison with producing point (focused) defects by pointfocusing of a laser onto the surface or else into the interior of thesubstrate material and subsequent breaking after setting such pointfocuses at different depths of the material, embodiments describedherein have in particular the advantage that a much higher aspect ratioof the cut can be achieved. Problems of such known methods that occur onaccount of scarcely directed crack formation, in particular in the caseof thicker substrates, are consequently avoided. The machining speed isalso increased by a multiple, in particular in the case of thickersubstrates (in the case of which it is necessary to set at a definedposition in the plane of the substrate multiple points of damage atdifferent depths of the substrate, from the upper side to the undersideof the substrate).

Ablation at the surface, flash formations at the surface and particleformations are avoided (the latter in particular if the position of thefocal line in relation to the substrate is set such that the extendedinduced absorption and crack formation from the surface of the substrateis into the interior of the substrate). In this case, the first (wanted)damage consequently takes place directly at the surface and continues ina defined way along the crack formation zone into the depth of thesubstrate by induced absorption.

Various materials, in particular glass sheets, sapphire sheets,semiconductor wafers, . . . can be machined. In this respect, bothindividual layers of corresponding materials and laminates (stacks ofmultiple individual substrate layers) can be machined. The focal linemay in this case be positioned and aligned such that, even in theinterior of a stack of layers, only a defined layer is separated.Various sandwich structures of stacks of layers can be machined:glass-air-glass composites, glass-film-glass composites, glass-glasscomposites. Consequently, the selective cutting of individual layerseven within a stack is possible, as is the severing of intermediatelayers (for example films or adhesive film).

Already coated materials (for example AR coated, TCO coated) or elsesubstrates non-transparently printed on one side can also be machinedand separated.

Free-form cuts are possible, without the geometry being restricted bythe crack formation in the substrate. Consequently, virtually anydesired free-form cuts can be introduced into transparent media (thecutting direction is not direction-dependent). Consequently, obliquecuts can be introduced into the substrate, for example with angles ofadjustment which, on the basis of the normal, have angles of up to β=30°or β=45°.

Cutting is possible virtually without any cutting gap: only materialdamage is produced, generally of an extent in the range between 1 and 10μm. In particular, no cutting loss with respect to material or surfacearea is thereby generated. This is advantageous in particular whencutting semiconductor wafers, since cutting gap losses would reduce theactively usable surface area of the wafer. The method of focal linecutting described herein consequently produces an increased surface areayield. The absence of material loss is advantageous in particular alsowhen cutting precious stones (for example diamond); though the area ofuse described herein is preferably the cutting or separating ofsheet-like substrates, non-sheet-like substrates or workpieces can alsobe machined.

The method described herein may also be used in particular in thein-line operation of production processes. This takes place particularlyadvantageously in the case of production processes that proceed by aroll-to-roll method.

Single-pulse lasers may be used as well as lasers that generate burstpulses. In principle, the use of lasers in continuous-wave operation isalso conceivable.

The following specific areas of application arise by way of example:

1. Separating sapphire LEDs with the possibility of fully or partiallycutting the sapphire wafer. In this case, the metal layer may likewisebe severed at the same time by the method described herein, doing so ina single step.

2. The individual separation of semiconductor wafers is possible withoutdamaging the tape. For this purpose, the focal line is only partiallytaken into the interior of the substrate material, so that it begins atthe surface and stops before the taped film (on the rear surface of thesubstrate that is facing away from the laser): for example, about 10% ofthe material is not separated. The film consequently remains intactbecause the focal line “stops” before the film. The semiconductor wafercan then subsequently be separated over the remaining 10% by way ofmechanical forces (or thermal forces, see the following example with theCO₂ laser).

3. Cutting of coated materials: examples here are Bragg reflectors (DBR)or else metal-coated sapphire wafers. Processed silicon wafers, to whichthe active metal or metal-oxide layers have already been applied, canalso be cut according to the invention. Other examples are the machiningof ITO or AlZnO, by which substrates that are required for example forproducing touchscreens or smart windows are coated. On account of thevery extended focal line (in comparison with its diameter), part of thefocal line will remove the metal layer (or another layer), while therest of the focal line penetrates into the transparent material and cutsit. This also has the advantage in particular that correspondinglycoated substrates can be separated in a one-step process, that is to sayin a process in which the coating and the substrate are separated in oneoperation.

4. The cutting of very thin materials (for example substrates of glasswith thicknesses of less than 300 μm, less than 100 μm or even less than50 μm) is particularly advantageous. These materials can only bemachined very laboriously by conventional mechanical methods. Indeed, inthe case of the mechanical methods, edges, damage, cracks or spallingoccur, which either make the substrates unusable or necessitatelaborious re-working operations. By contrast, in the case of thinmaterials, the cutting described herein offers the advantages inparticular of avoiding edge damage and cracks, so that no re-working isnecessary, of very high cutting speeds (>1 m/s), of a high yield and ofcarrying out the process in a single step.

5. The method described herein can also be used in particular in theproduction of thin film glasses, which are produced by a continuouslyrunning glass-pulling process, for trimming the edges of the film.

FIG. 2 diagrammatically shows the basic procedure of the machiningmethod according to the invention. A laser beam 2, which is emitted bythe laser 3 not shown here (see FIG. 7) and is denoted on the beam inputside of the optical arrangement 6 by the reference sign 2 a, is directedonto the optical arrangement 6 (see the following exemplary embodimentsof this). The optical arrangement 6 forms from the radiated-in laserbeam on the beam output side an extended laser beam focal line 2 b overa defined range of extent along the direction of the beam (length 1 ofthe focal line). The substrate 1 to be machined, here a sheet-likesubstrate 1, is positioned after the optical arrangement in the path ofrays, at least a portion thereof coinciding with the laser beam focalline 2 b of the laser radiation 2. The reference sign 1 a denotes thesurface of the sheet-like substrate that is facing the opticalarrangement 6 or the laser, the reference sign 1 b denotes the rearsurface 1 b of the substrate 1, at a distance from and usually parallelto said first surface. The substrate thickness (perpendicularly to thesurfaces 1 a and 1 b, that is to say measured in relation to the planeof the substrate) is denoted here by the reference sign d.

As FIG. 2 a shows, here the substrate 1 is aligned perpendicularly tothe longitudinal axis of the beam and consequently to the focal line 2 bproduced by the optical arrangement 6 in space downstream of the same(the substrate is perpendicular to the plane of the drawing) and, seenalong the direction of the beam, positioned in relation to the focalline 2 b such that, seen in the direction of the beam, the focal line 2b begins before the surface 1 a of the substrate and ends before thesurface 1 b of the substrate, that is to say still within the substrate.Consequently (with suitable laser intensity along the laser beam focalline 2 b, which is ensured by the focusing of the laser beam 2 on aportion of the length 1, that is to say by a line focus of the length1), the extended laser beam focal line 2 b produces in the region ofcoincidence of the laser beam focal line 2 b with the substrate 1, thatis to say in the material of the substrate that is passed over by thefocal line 2 b, an extended portion 2 c, seen along the longitudinaldirection of the beam, along which an induced absorption is produced inthe material of the substrate, which induces a crack formation in thematerial of the substrate along the portion 2 c. The crack formationtakes place in this case not only locally but over the entire length ofthe extended portion 2 c of the induced absorption. The length of thisportion 2 c (that is to say ultimately the length of the coincidence ofthe laser beam focal line 2 b with the substrate 1) is provided herewith the reference sign L. The average diameter or the average extent ofthe portion of the induced absorption (or of the regions in the materialof the substrate 1 that are subjected to the crack formation) is denotedhere by the reference sign D. This average extent D corresponds heresubstantially to the average diameter δ of the laser beam focal line 2b.

As FIG. 2 a shows, consequently, substrate material that is transparentto the wavelength λ of the laser beam 2 is heated by induced absorptionalong the focal line 2 b. FIG. 2 b diagrammatically shows how the heatedmaterial ultimately expands, so that a correspondingly induced stressleads to the microcrack formation, the stress being greatest at thesurface 1 a.

Actual optical arrangements 6 that can be used for producing the focalline 2 b, and also an actual optical setup (FIG. 7), in which theseoptical arrangements can be used, are described below. All of thearrangements and setups are based here on the descriptions given above,so that identical reference signs are used in each case for componentsor features that are identical or correspond in their function.Therefore, only the differences are respectively described below.

Since the separating surface ultimately leading to the separation is, oris intended to be, of high-quality (with regard to rupture strength, thegeometrical precision, roughness and the avoidance of re-workingrequirements), the individual focal lines to be positioned along theseparating line 5 on the surface of the substrate should be produced asdescribed with the following optical arrangements (the opticalarrangement is alternatively also referred to hereinafter as laseroptics). The roughness results here in particular from the spot size orthe spot diameter of the focal line. In order with a given wavelength λof the laser 3 (interaction with the material of the substrate 1) to beable to achieve a small spot size, of for example 0.5 μm to 2 μm,generally certain requirements have to be imposed on the numericalaperture of the laser optics 6. These requirements are satisfied by thelaser optics 6 described below.

To achieve the desired numerical aperture, on the one hand the opticsmust have the necessary aperture at a given focal length, according tothe known formulae given by Abbé (N.A.=n sin (theta), n: refractiveindex of the glass to be machined, theta: half the angular aperture; andtheta=arctan (D/2 f); D: aperture, f: focal length). On the other hand,the laser beam must illuminate the optics up to the necessary aperture,which is typically accomplished by beam expansion by means of expansiontelescopes between the laser and the focusing optics.

The spot size should at the same time not vary too much, for a uniforminteraction along the focal line. This can be ensured for example (seeexemplary embodiment below) by the focusing optics only beingilluminated in a narrow, annular region, in that then of course the beamaperture, and consequently the numerical aperture, change only by asmall amount in percentage terms.

According to FIG. 3 a (section perpendicularly to the plane of thesubstrate at the height of the central ray in the bundle of laser raysof the laser radiation 2; here, too, the radiating in of the laser beam2 takes place perpendicularly to the plane of the substrate, i.e. theangle β is 0°, so that the focal line 2 b or the extended portion of theinduced absorption 2 c is parallel to the normal to the substrate), thelaser radiation 2 a emitted by the laser 3 is initially directed onto acircular diaphragm 8, which is completely nontransparent to the laserradiation used. The diaphragm 8 is in this case oriented perpendicularlyto the longitudinal axis of the beam and centered on the central ray ofthe bundle of rays 2 a shown. The diameter of the diaphragm 8 is chosensuch that the bundles of rays lying close to the center of the bundle ofrays 2 a or the central ray (denoted here by 2 aZ) impinge on thediaphragm and are completely absorbed by it. Only rays lying in theouter peripheral region of the bundle of rays 2 a (peripheral rays,denoted here by 2 aR) are not absorbed on account of the reduceddiaphragm size in comparison with the beam diameter, but pass by thediaphragm 8 laterally and impinge on the peripheral regions of thefocusing optical element of the optical arrangement 6, formed here as aspherically ground, biconvex lens 7.

The lens 7, centered on the central ray, is deliberately formed here asa non-corrected, biconvex focusing lens in the form of a customaryspherically ground lens. In other words, the spherical aberration ofsuch a lens is deliberately utilized. As an alternative to this,aspheric lenses or multilens systems that deviate from ideally correctedsystems and specifically do not form an ideal focal point but adefinite, elongated focal line of a defined length may also be used(that is to say lenses or systems that specifically no longer have asingle focal point). The zones of the lens consequently focusspecifically in dependence on the distance from the center of the lensalong a focal line 2 b. Here, the diameter of the diaphragm 8transversely to the direction of the beam is approximately 90% of thediameter of the bundle of rays (the diameter of the bundle of rays isdefined by the extent up to decay to 1/e) and about 75% of the diameterof the lens of the optical arrangement 6. Consequently, the focal line 2b of a non-aberration-corrected spherical lens 7 that has been producedby blocking out the bundle of rays in the middle is used herein. Thesection in a plane through the central ray is represented; the completethree-dimensional bundle is obtained when the rays represented arerotated about the focal line 2 b.

A disadvantage of this focal line is that the conditions (spot size,intensity of the laser) change along the focal line, and consequentlyalong the desired depth in the material, and consequently the desiredtype of interaction (no significant melting, induced absorption,thermal-plastic deformation up to crack formation) can possibly only beset within part of the focal line. This conversely means that possiblyonly part of the radiated-in laser light is absorbed in the way desired.Consequently, on the one hand the efficiency of the method (necessaryaverage laser power for the desired separating speed) is impaired, onthe other hand under some circumstances laser light is transmitted toundesired, deeper-lying locations (parts or layers adhering to thesubstrate, or to the substrate holder) and interacts there in anundesired way (heating, dispersion, absorption, undesired modification).

FIG. 3 b shows (not only for the optical arrangement in FIG. 3 a, but inprinciple also for all other optical arrangements 6 that can be used)that the laser beam focal line 2 b can be positioned variously bysuitable positioning and/or alignment of the optical arrangement 6 inrelation to the substrate 1 and by suitable choice of the parameters ofthe optical arrangement 6: as the first line from FIG. 3 bdiagrammatically shows, the length 1 of the focal line 2 b may be setsuch that it exceeds the substrate thickness d (here by a factor of 2).Consequently, if the substrate 1 is placed centrally in relation to thefocal line 2 b, seen in the longitudinal direction of the beam, anextended portion of induced absorption 2 c is produced over the entiresubstrate thickness d.

In the case shown in the second line in FIG. 3 b, a focal line 2 b ofthe length 1 that corresponds approximately to the extent of thesubstrate d is produced. Since the substrate 1 is positioned in relationto the line 2 such that the line 2 b begins at a point before, that isto say outside, the substrate, the length L of the extended portion ofinduced absorption 2 c (which extends here from the surface of thesubstrate to a defined depth of the substrate, but not as far as therear surface 1 b) is less here than the length 1 of the focal line 2 b.The third line in FIG. 3 b shows the case in which the substrate 1 ispositioned partially before the beginning of the focal line 2 b, seenalong the direction of the beam, so that here, too, 1>L applies for thelength 1 of the line 2 b (L=extent of the portion of induced absorption2 c in substrate 1). The focal line consequently begins in the interiorof the substrate and extends beyond the rear surface 1 b to outside thesubstrate. The fourth line in FIG. 3 b finally shows the case in whichthe focal line length 1 produced is less than the substrate thickness d,so that—with central positioning of the substrate in relation to thefocal line seen in the direction of irradiation—the focal line beginshere close to the surface 1 a in the interior of the substrate and endsclose to the surface 1 b in the interior of the substrate (1=0.75·d).

It is particularly advantageous here to realize the focal linepositioning such that at least one of the surfaces 1 a, 1 b is passedover by the focal line; the portion of the induced absorption 2 cconsequently begins at at least one surface. In this way, virtuallyideal cuts can be achieved by avoiding ablation, flash and particleformation at the surface.

FIG. 4 shows a further optical arrangement 6 that can be used. The basicsetup follows that described in FIG. 3 a, so that only the differencesare described below. The optical arrangement shown is based on the ideaof using optics with a non-spherical free surface which is shaped suchthat a focal line of a defined length 1 is formed for the formation ofthe focal line 2 b. For this purpose, aspheric lenses may be used asoptical elements of the optical arrangement 6. For example, in FIG. 4, aso-called cone prism, which is often also referred to as an axicon, isused. An axicon is a special, conically ground lens that forms a pointsource on a line along the optical axis (or else annularly transforms alaser beam). The setup of such an axicon is known in principle to aperson skilled in the art. Here, the cone angle is for example 10°. Theaxicon denoted here by the reference sign 9 is aligned with its cone tipcounter to the direction of irradiation and centered on the center ofthe beam. Since the focal line 2 b of the axicon 9 already begins withinthe same, the substrate 1 (which is arranged here perpendicularly to theaxis of the principal ray) may be positioned in the path of raysdirectly after the axicon 9. As FIG. 4 shows, on account of the opticalproperties of the axicon, a displacement of the substrate 1 along thedirection of the beam is also possible without it leaving the region ofthe focal line 2 b. The extended portion of the induced absorption 2 cin the material of the substrate 1 consequently extends over the entiresubstrate depth d.

However, the setup shown has the following restrictions: since the focalline of the axicon 9 already begins within the lens, with a finiteworking distance between the lens and the material, a significant partof the laser energy is not focused into the part 2 c of the focal line 2b that lies in the material. Furthermore, with the available refractiveindex and cone angles of the axicon 9, the length 1 of the focal line 2b is linked to the beam diameter, for which reason, in the case ofrelatively thin materials (a few millimeters), the focal line isaltogether too long, as a result of which in turn the laser energycannot be specifically focused into the material.

For this reason, an improved optical arrangement 6 is obtained if itcomprises both an axicon and a focusing lens. FIG. 5 a shows such anoptical arrangement 6, in which there is positioned on the path of raysof the laser 3, seen along the direction of the beam, firstly a firstoptical element with a non-spherical free surface, which is shaped forthe forming of an extended laser beam focal line 2 b. In the case shown,this first optical element is an axicon 10 with a 5° cone angle, whichis positioned perpendicularly to the direction of the beam and centeredon the laser beam 3. The cone tip of the axicon in this case pointscounter to the direction of the beam. Positioned at the distance z1 fromthe axicon 10 in the direction of the beam is a second, focusing opticalelement, here a plano-convex lens 11 (the convexity of which facestoward the axicon). The distance z1 is chosen here to be about 300 mm,such that the laser radiation formed by the axicon 10 impinges in anannular manner on the outer regions of the lens 11. The lens 11 focusesthe annularly impinging radiation on the beam output side onto a focalline 2 b of a defined length, of here 1.5 mm, at the distance z2, ofhere about 20 mm, from the lens 11. Here, the effective focal length ofthe lens 11 is 25 mm. The annular transformation of the laser beam bythe axicon 10 is provided here with the reference sign SR.

FIG. 5 b shows in detail the formation of the focal line 2 b and of theinduced absorption 2 c in the material of the substrate 1 according toFIG. 5 a. The optical properties of the two elements 10, 11 and thepositioning of the same here are such that the extent 1 of the focalline 2 b in the direction of the beam coincides exactly with thethickness d of the substrate 1. Accordingly, an exact positioning of thesubstrate 1 along the direction of the beam is necessary in order toposition the focal line 2 b exactly between the two surfaces 1 a and 1 bof the substrate 1, as shown in FIG. 5 b.

It is consequently advantageous if the focal line is formed at a certaindistance from the laser optics, and the large part of the laserradiation is focused up to a desired end of the focal line. Asdescribed, this can be achieved by a mainly focusing element 11 (lens)only being illuminated annularly on a desired zone, whereby on the onehand the desired numerical aperture is realized, and consequently thedesired spot size, but on the other hand, after the desired focal line 2b, the circle of least diffusion loses intensity after a very shortdistance in the middle of the spot, since a substantially annular spotforms. Consequently, the crack formation is stopped within a shortdistance at the desired depth of the substrate. A combination of theaxicon 10 and the focusing lens 11 satisfies this requirement. Here, theaxicon 10 acts in two ways: a usually round laser spot is sent by theaxicon 10 annularly onto the focusing lens 11 and the asphericity of theaxicon 10 has the effect that, instead of a focal point in the focalplane of the lens, a focal line forms outside the focal plane. Thelength 1 of the focal line 2 b can be set by way of the beam diameter onthe axicon. The numerical aperture along the focal line can in turn beset by way of the distance z1 between the axicon and the lens and by wayof the cone angle of the axicon. In this way, the entire laser energycan consequently be concentrated in the focal line.

If the crack formation is intended to stop before the exit side of thesubstrate, the annular illumination still has the advantage that on theone hand the laser power is used in the best possible way, since a largepart of the laser light remains concentrated in the desired length ofthe focal line, on the other hand, due to the annular illuminated zonetogether with the desired aberration set by the other optical functions,a uniform spot size along the focal line can be achieved, andconsequently a uniform separating process along the focal line can beachieved.

Instead of the plano-convex lens shown in FIG. 5 a, a focusing meniscuslens or some other higher corrected focusing lens (aspheric lens,multilens system) may also be used.

To produce very short focal lines 2 b with the combination shown in FIG.5 a of an axicon and a lens, very small beam diameters of the laser beamincident on the axicon would have to be chosen. This has the practicaldisadvantage that the centering of the beam on the tip of the axiconmust be very exact, and therefore the result is very sensitive todirectional fluctuations of the laser (beam drift stability).Furthermore, a narrowly collimated laser beam is very divergent, i.e.the bundle of rays scatters again over short distances on account of thediffraction of light.

Both can be avoided by inserting a further lens, a collimation lens 12(FIG. 6): this further positive lens 12 allows the annular illuminationof the focusing lens 11 to be set very narrowly. The focal length f′ ofthe collimation lens 12 is chosen such that the desired ring diameter dris obtained when there is a distance z1 a from the axicon to thecollimation lens 12 that is equal to f′. The desired width br of thering can be chosen by way of the distance z1 b (collimation lens 12 tofocusing lens 11). Purely geometrically, a short focal line then followsfrom the small width of the annular illumination. A minimum is in turnachieved at the distance f′.

The optical arrangement 6 shown in FIG. 6 is consequently based on thatshown in FIG. 5 a, so that only the differences are described below. Inaddition, the collimation lens 12, which is likewise formed here as aplano-convex lens (with its convexity pointing counter to the directionof the beam), has been introduced here centrally into the path of raysbetween the axicon 10 (which is arranged here with its cone tip counterto the direction of the beam) on the one hand and the plano-convex lens11 on the other hand. The distance of the collimation lens 12 from theaxicon 10 is denoted here by z1 a, the distance of the focusing lens 11from the collimation lens 12 is denoted by z1 b and the distance of thefocal line 2 b produced from the focusing lens 11 is denoted by z2 (seenin each case in the direction of the beam). As FIG. 6 shows, the annularradiation SR that is formed by the axicon and incident upon thecollimation lens 12 in a divergent manner and with the ring diameter dr,has the ring diameter dr remaining at least approximately constant alongthe distance z1 b and is set to the desired ring width br at thelocation of the focusing lens 11. In the case shown, a very short focalline 2 b is intended to be produced, so that the ring width br of about4 mm at the location of the lens 12 is reduced by the focusingproperties of the latter at the location of the lens 11 to about 0.5 mm(ring diameter dr here for example 22 mm).

In the example represented, with a typical beam diameter from the laserof 2 mm, with a focusing lens 11 of f=25 mm focal length and acollimation lens of f′=150 mm focal length, a length of the focal line 1of below 0.5 mm can be achieved. Furthermore, Z1 a=Z1 b=140 mm and Z2=15mm.

An example of the severing of unhardened glass with an opticalarrangement according to FIG. 3 a in a setup according to FIG. 7 isgiven below (instead of the optical arrangement 6 shown in FIG. 3 a, theother optical arrangements 6 described above may also be used in thesetup according to FIG. 7, in that the diaphragm-lens combination 8, 7shown there is correspondingly replaced).

Borosilicate or soda-lime glasses 1 without other colorations (inparticular with a low iron content) are optically transparent from about350 nm to about 2.5 μm. Glasses are generally poor heat conductors, forwhich reason even laser pulse durations of a few nanoseconds do notallow any significant heat diffusion out of a focal line 2 b.Nevertheless, even shorter laser pulse durations are advantageous, sincewith sub-nanosecond or picosecond pulses a desired induced absorptioncan be achieved more easily by way of non-linear effects (intensity muchhigher).

Suitable for example for severing flat glasses is a commerciallyavailable picosecond laser 3, which has the following parameters:wavelength 1064 nm, pulse duration of 10 picoseconds, pulse repetitionrate of 100 kHz, average power (measured directly after the laser) of upto 50 W. The laser beam initially has a beam diameter (measured at 13%of the peak intensity, i.e. 1/e² diameter of a Gaussian bundle of rays)of about 2 mm, the beam quality is at least M²<1.2 (determined inaccordance with DIN/ISO 11146). With beam expanding optics 22(commercially available Kepler beam telescope), the beam diameter isincreased by a factor of 10 to about 20-22 mm (21, 23, 24 and 25 arebeam-deflecting mirrors). With a so-called annular diaphragm 8 of 9 mmin diameter, the inner part of the bundle of rays is cut off, so that anannular beam forms. With this annular beam, a plano-convex lens 7 with a28 mm focal length (quartz glass with a radius of 13 mm) is illuminatedfor example. The strong (desired) spherical aberration of the lens 7 hasthe effect of producing the focal line. See in this respect not onlyFIG. 7 but also FIG. 8, which diagramatically shows the production ofthe focal line 2 b from the peripheral rays by the lens 7.

The theoretical diameter δ of the focal line varies along the axis ofthe beam, and is therefore advantageous for producing a homogeneouscrack surface if the substrate thickness d here is smaller than about 1mm (typical thicknesses for display glasses are 0.5 mm to 0.7 mm) A spotsize of about 2 μm and a spacing from spot to spot of 5 μm give a speedat which the focal line can be passed 5 over the substrate 1 (see FIG.9) of 0.5 m/sec. With an average power on the substrate of 25 W(measured after the focusing lens 7), the pulse repetition rate of 100kHz gives a pulse energy of 250 μJ, which may also take place in astructured pulse (rapid sequence of single pulses at intervals of only20 ns, known as a burst pulse) of 2 to 5 sub-pulses.

Unhardened glasses have substantially no internal stresses, for whichreason, without any external action, the zone of disturbance, which isstill interlinked and connected by unseparated bridges, at first stillholds the parts together here. If, however, a thermal stress isintroduced, the substrate separates completely, and without any furtherforce being introduced externally, along the lasered rupture surface 5.For this purpose, a CO₂ laser with an average power of up to 250 W isfocused onto a spot size of about 1 mm, and this spot is passed over theseparating line 5 at up to 0.5 m/s. The local thermal stress due to thelaser energy introduced (5 J per cm of the separating line 5) separatesthe workpiece 1 completely.

For separating thicker glasses, the threshold intensity for the process(induced absorption and formation of a zone of disturbance by thermalshock) must of course be reached over a longer focal line 1. Highernecessary pulse energies and higher average power outputs consequentlyfollow. With the optics setup described above and the maximum laserpower available (after losses through optics) of 39 W on the substrate,the severing of glass about 3 mm thick is successfully achieved. In thiscase, on the one hand the annular diaphragm 8 is removed, and on theother hand the distance of the lens 7 from the substrate is corrected(increased in the direction of nominal focal distance) such that alonger focal line is produced in the substrate.

A further exemplary embodiment of severing hardened glass (likewise withthe device shown in FIGS. 3 a and 7) is presented below.

Sodium-containing glasses are hardened, in that sodium is exchanged forpotassium at the surface of the glass by immersion in baths of liquidpotassium salt. This leads to a considerable internal stress(compressive stress) in a layer 5-50 μm thick at the surfaces, which inturn leads to the greater stability.

In principle, the process parameters when severing hardened glasses aresimilar to those for unhardened glasses of comparable dimensions andcomposition. However, the hardened glass can break very much more easilydue to the internal stress, specifically due to undesired crack growth,which does not take place along the lasered intended rupture surface 5,but into the material. Therefore, there is a narrower parameter fieldfor the successful severing of a specific hardened glass. In particular,the average laser power and the associated cutting speed must bemaintained very precisely, specifically in dependence on the thicknessof the hardened layer. For a glass with a hardened layer 40 μm thick anda total thickness of 0.7 mm, the following parameters are obtained forexample in the case of the aforementioned setup: a cutting speed of 1m/s at a pulse repetition rate of 100 kHz, and therefore a spot spacingof 10 μm, with an average power of 14 W.

The internal stress of the hardened glasses has the effect that therupture zone 5 forms completely after a little time (a few seconds), andthe substrate is separated into the desired parts.

Very thin hardened glasses (<100 μm) consist predominantly of toughenedmaterial, i.e. the front and rear sides are reduced in their sodiumcontent, and consequently hardened, in each case by for example 30 μm,and only 40 μm in the interior are unhardened. This material breaks veryeasily and completely if one of the surfaces is damaged. It has so farnot been possible in the prior art for such hardened glass films to bemachined.

The severing of this material by the method described herein issuccessfully achieved if a) the diameter of the focal line is verysmall, for example less than 1 μm, b) the spacing from spot to spot issmall, for example between 1 and 2 μm, and c) the separating speed ishigh enough for the crack growth not to get ahead of the laser process(high laser pulse repetition rate of for example 200 kHz at 0.2 to 0.5m/s).

A further exemplary embodiment (likewise with the device described inFIGS. 3 a and 7) for severing sapphire glass and crystalline sapphire ispresented below.

Sapphire crystals and sapphire glasses are glasses which, thoughoptically similar (transparency and refractive index), behave verydifferently mechanically and thermally. For instance, sapphire is anexcellent heat conductor, can withstand extreme mechanical loading andis very hard and scratch-resistant. Nevertheless, with the laser andoptics setup described above, thin (0.3 mm to 0.6 mm) sapphire crystalsand glasses can be severed. Because of the great mechanical stability,it is particularly important that the remaining bridges between theparts to be separated are minimized, since otherwise very high forcesare required for ultimate separation. The zone of disturbance must beformed as completely as possible from the entry surface 1 a to the exitsurface 1 b of the substrate. As in the case of thicker glasses, thiscan be achieved with higher pulse energy, and consequently higheraverage laser power. Furthermore, crystalline sapphire is birefringent.The cutting surface must lie perpendicularly to the optical axis(so-called C-cut). For severing a crystalline sapphire wafer of 0.45 mmin thickness, the following parameters can be used: an average laserpower of 30 W at a pulse repetition rate of 100 kHz, a spot size of 2μm, and a spot spacing of 5 μm, which corresponds to a cutting speed of0.5 m/s at the pulse repetition rate mentioned. As in the case of glass,complete separation may require subsequent heating of the cutting line 5to be carried out, for example with a CO₂ laser spot, in order that thethermal stress is used to make the zone of disturbance go through crackgrowth to form a complete, continuous, non-interlinked separatingsurface.

FIG. 9 finally shows a micrograph of the surface of a glass sheetmachined as described herein. The individual focal lines or extendedportions of induced absorption 2 c, which are provided here with thereference signs 2 c-1, 2 c-2 . . . (into the depth of the substrateperpendicularly to the surface represented), are joined together alongthe line 5, along which the laser beam has been passed over the surface4 of the substrate, by crack formation to form a separating surface forthe separation of the parts of the substrate. The multiplicity ofindividual extended portions of induced absorption can be seen well, inthe case shown the pulse repetition rate of the laser having been madeto match the rate of the advancement for moving the laser beam over thesurface 4 such that the ratio V3=a/δ of the average spacing a ofdirectly adjacent portions 2 c-1, 2, 2 c-2 . . . and the averagediameter δ of the laser beam focal line is approximately 2.0.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

The teachings of all patents, published applications and referencescited herein are incorporated by reference in their entirety.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method comprising: focusing a pulsed laser beaminto a laser beam focal line, viewed along the beam propagationdirection, the laser beam focal line having a length in a range ofbetween 0.1 mm and 100 mm; and directing the laser beam focal line intoa material at an angle of incidence to a surface of the material, thelaser beam focal line generating an induced absorption within thematerial, the induced absorption producing a material modification alongthe laser beam focal line within the material.
 2. The method of claim 1,further including translating the material and the laser beam relativeto each other, thereby producing a plurality of material modificationswithin the material, the material modifications spaced apart so as toseparate the material into at least two pieces.
 3. The method of claim1, wherein the laser beam has an average laser energy measured at thematerial less than about 400 μJ.
 4. The method of claim 3, wherein thelaser beam has an average laser energy measured at the material lessthan about 250 μJ.
 5. The method of claim 1, wherein the pulse durationis in a range of between greater than about 10 picoseconds and less thanabout 100 picoseconds.
 6. The method of claim 1, wherein the laser beamhas a pulse duration less than 10 picoseconds.
 7. The method of claim 1,wherein the pulse repetition frequency is in a range of between 10 kHzand 1000 kHz.
 8. The method of claim 7, wherein the pulse repetitionfrequency is in a range of between 10 kHz and 100 kHz.
 9. The method ofclaim 1, wherein the pulse repetition frequency is less than 10 kHz. 10.The method of claim 1, wherein the material is glass.
 11. The method ofclaim 1, wherein the material is sapphire.
 12. The method of claim 1,wherein the material is a semiconductor wafer.
 13. The method of claim1, wherein the material modification is crack formation.
 14. The methodof claim 1, wherein the angle of incidence of the laser beam focal lineis less than or equal to about 45 degrees to the surface of thematerial.
 15. The method of claim 1, wherein the angle of incidence ofthe laser beam focal line is perpendicular to the surface of thematerial.
 16. The method of claim 15, wherein the laser beam focal lineis contained entirely within the material, and the laser beam focal linedoes not extend to either surface of the material.
 17. The method ofclaim 15, wherein the material modification extends within the materialto at least one of two opposing surfaces of the material.
 18. The methodof claim 15, wherein the material modification extends within thematerial from one of two opposing surfaces of the material to the otherone of the two opposing surfaces, over the entire thickness of thematerial.
 19. The method of claim 15, wherein, for each laser pulse, thematerial modification extends within the material from one of twoopposing surfaces of the material to the other one of the two opposingsurfaces, over the entire thickness of the material.
 20. The method ofclaim 1, wherein the pulsed laser beam has a wavelength selected suchthat the material is substantially transparent at this wavelength. 21.The method of claim 1, wherein the wavelength is less than about 1.8 μm.22. The method of claim 1, wherein the laser beam focal line has anaverage spot diameter in a range of between 0.5 μm and 5 μm.
 23. Asystem comprising: a pulsed laser; and an optical assembly positioned inthe beam path of the laser, configured to transform the laser beam intoan laser beam focal line, viewed along the beam propagation direction,on the beam emergence side of the optical assembly, the laser beam focalline having a length in a range of between 0.1 mm and 100 mm, theoptical assembly including a focusing optical element with sphericalaberration configured to generate the laser beam focal line, said laserbeam focal line adapted to generate an induced absorption within amaterial, the induced absorption producing a material modification alongthe laser beam focal line within the material.
 24. The system of claim23, wherein the laser beam has an average laser energy measured at thematerial less than about 400 μJ.
 25. The system of claim 24, wherein thelaser beam has an average laser energy measured at the material lessthan about 250 μJ.
 26. The system of claim 23, wherein the pulseduration is in a range of between greater than about 10 picoseconds andless than about 100 picoseconds.
 27. The system of claim 23, wherein thelaser beam has a pulse duration less than 10 picoseconds.
 28. The systemof claim 23, wherein the pulse repetition frequency is in a range ofbetween 10 kHz and 1000 kHz.
 29. The system of claim 28, wherein thepulse repetition frequency is in a range of between 10 kHz and 100 kHz.30. The system of claim 23, wherein the pulse repetition frequency isless than 10 kHz.
 31. The system of claim 23, wherein the opticalassembly includes an annular aperture positioned in the beam path of thelaser before the focusing optical element, the annular apertureconfigured to block out one or more rays in the center of the laser beamso that only marginal rays outside the center incide on the focusingoptical element, and thereby only a single laser beam focal line, viewedalong the beam direction, is produced for each pulse of the pulsed laserbeam.
 32. The system of claim 23, wherein the focusing optical elementis a spherically cut convex lens.
 33. The system of claim 32, whereinthe focusing optical element is a conical prism having a non-sphericalfree surface.
 34. The system of claim 33, wherein the conical prism isan axicon.
 35. The system of claim 23, wherein the optical assemblyfurther includes a second optical element, the two optical elementspositioned and aligned such that the laser beam focal line is generatedon the beam emergence side of the second optical element at a distancefrom the second optical element.
 36. The system of claim 23, wherein thepulsed laser beam has a wavelength selected such that the material issubstantially transparent at this wavelength.
 37. The system of claim36, wherein the wavelength is less than about 1.8 μm.
 38. The system ofclaim 23, wherein the laser beam focal line has an average spot diameterin a range of between 0.5 μm and 5 μm.
 39. The system of claim 23,wherein the material is glass.
 40. The system of claim 23, wherein thematerial is sapphire.
 41. The system of claim 23, wherein the materialis a semiconductor wafer.
 42. The system of claim 23, wherein thematerial modification is crack formation.
 43. A glass article comprisingat least one surface having a plurality of material modifications alongthe surface, each material modification having a length in a range ofbetween 0.1 mm and 100 mm, and an average diameter in a range of between0.5 μm and 5 μm.
 44. A glass article comprising at least one surfacehaving a plurality of material modifications along the surface, eachmaterial modification having a ratio V3=a/δ of the average distance a ofthe directly neighboring material modifications and the average diameterδ of a laser beam focal line that created the material modificationsequal to approximately 2.0.